1. Field of the Invention
The present invention relates, generally, to solenoid actuated valves and, more specifically, to self-latching solenoid actuated valves and control circuits for operating same.
2. Description of the Related Art
Solenoids are well known electromechanical devices used to convert electrical energy into mechanical energy and particularly into short stroke mechanical motion. As such, solenoids have long been employed to actuate valves in response to an electrical signal. For example, it is known in the related art to employ a solenoid to bias a valve member in one direction against the biasing force of a return spring. When power to the solenoid is interrupted, the return spring biases the valve member back to its first position.
In certain applications, valve members must be positively maintained in different predetermined positions to control the flow of fluids, such as air, through the valve. One embodiment employed in the related art to achieve this results eliminates the return spring employed to bias the valve member to a specific location and replaces it with a second solenoid. The second solenoid is powered to positively move the valve member to a predetermined position and maintain it there until the second solenoid is de-energized and the first solenoid is energized to move the valve member back to the other position. However, this approach suffers from the disadvantage that double solenoid operated valves increase the size, weight, cost and complexity of the valve. Further, in the case of poppet-type valves, at least one coil must be powered at all times to ensure that the valve member is properly seated in the predetermined position. An unexpected, inadvertent or even planned shut down of power to the solenoid results in a loss of control of the valve. Additionally, in applications where the efficiency of the solenoid is of concern, such as where there is a limited source of electrical power, solenoids which must be continually powered to hold a valve member in a specific position or double solenoid actuated valves are generally unacceptable.
To decrease the power dissipated by the solenoid, and particularly in applications where the solenoid is to be retained in the actuated position for significant time periods, latching mechanisms are employed in the related art to hold the mechanical output of the solenoid in one position or the other without continuous power required to the solenoid. To this end, conventional self-latching solenoids known in the related art typically employ a movable pole piece and a fixed permanent magnet which are subject to an electromagnetic flux to bias a valve member. Usually, current flowing through the coil in one direction causes the pole piece to move away from the permanent magnet and to be attracted to another stationary element in the solenoid thereby actuating the valve member. Power to the coil is then interrupted but the latent magnetic force acting on the movable pole piece causes it to remain magnetically attracted to the stationary portion of the solenoid or "latched" in its last position.
Control circuits are used to reverse the direction of current through the solenoid coil thereby reversing the direction of electromagnetic flux. Reversing the direction of current through the coil reverses the "polarity" of the movable pole piece, driving it in the opposite direction toward the permanent magnet where it again becomes "latched" after the power to the solenoid has been interrupted. The return spring is then typically free to bias the valve member in the opposite direction. In this way, the valve member may be moved to, and maintained in, any predetermined position by actuation of the solenoid after a relatively short pulse of electrical current through the solenoid coil.
While the self-latching solenoid actuated valves known in the related art have generally worked well for their intended purposes, there continues to be a need for smaller, faster acting self-latching solenoid actuated valves having low power consumption. This is especially true for small pneumatic valves used, for example, to control small air cylinders. In addition, there continues to be a need for control circuits which result in lower power consumption than those circuits known in the related art.